Field of the Invention
The present invention relates to a protective film detecting apparatus and method for detecting whether or not a protective film is properly formed on a work surface of a workpiece.
Description of the Related Art
As a method for dividing a workpiece such as a semiconductor wafer and an optical device wafer along streets formed on the workpiece, there has been proposed a method including the steps of applying a pulsed laser beam to the workpiece along the streets to thereby form a laser processed groove on the workpiece along each street and then breaking the wafer along each laser processed groove by using a mechanical breaking apparatus (see Japanese Patent Laid-open No. 1998-305420, for example). In this method, the laser beam is applied along each street, so that heat energy is concentrated at an area where the laser beam is applied, causing the generation of debris. Accordingly, there is a problem such that the debris may adhere to the front side of each device formed on the workpiece, causing a reduction in quality of each device.
To solve this problem, there has been proposed a laser processing apparatus capable of forming a protective film of polyvinyl alcohol etc. on one side (work surface) of the workpiece to which a laser beam is to be applied and applying the laser beam through the protective film to the workpiece (see Japanese Patent Laid-open No. 2007-201178, for example). This laser processing apparatus includes a nozzle for supplying a liquid resin to the workpiece, wherein the liquid resin is dropped from the nozzle to the workpiece and the workpiece is then rotated to thereby form the protective film over the work surface of the workpiece.
This protective film must be formed so as to cover a portion of the work surface where the debris may adhere to cause a reduction in quality of each device. However, there is a possibility that the liquid resin may be solidified and stick to the nozzle or bubbles may be mixed in the liquid resin, causing the production of an uncoated area where the protective film is not formed. In this uncoated area, the debris may adhere to the work surface to cause a reduction in quality of each device. Accordingly, after forming the protective film, it is necessary to grasp whether or not the protective film has been completely formed on the work surface of the workpiece without the uncoated area. As a method for confirming the condition of the protective film formed on the work surface of the workpiece, there is a method including the steps of spraying steam to the work surface coated with the protective film, applying light to the work surface, and detecting scattering of the light due to asperities formed by the steam deposited to the uncoated area.